The present invention relates generally to the conversion of radiant energy into electrical signals, and more specifically to a photocapacitive MIS (metal-insulator-semiconductor) image converter.
In the field of image conversion, there are several devices currently in use. The vidicon is possibly the most widely used of these devices. It utilizes an electron beam to scan a photoconductive target which is the light sensor. A transparent conductive layer applied to the front side of the photoconductor serves as the signal electrode. The signal electrode is operated at a positive voltage with respect to the back side of the photoconductor which operates at near zero voltage as the cathode. In operation, a scanning beam initially charges the back side of the target to cathode potential. When a light pattern (image) is focused on the photoconductor, its conductivity increases in the illuminated areas and the back side of the target charges to more positive values. The scanning electron beam then reads the signal by depositing electrons on the positive charge areas providing a capacitively coupled signal output at the signal electrode. The vidicon is only moderately sensitive and response speed is comparatively slow.
The image orthicon utilizes a photocathode as the light sensor. The photoelectron image pattern developed at the photocathode is focused by an axial magnetic field producing one spiral loop onto a thin moderately insulating target surface. When the photoelectrons from the photocathode strike the target secondary emission occurs causing the establishment of net positive charges on the target. The electron beam scans the charged target pattern, deposits electrons on the more positively charged areas, and the modulated beam returns to an electron multiplier surrounding the electron gun. The output signal is the amplified anode current of the electron multiplier. The image orthicon has high sensitivity and can handle a wide range of light levels and contrasts, but is an intricate device and has high noise problems in the dark areas of the image.
The image isocon is a variant of the image orthicon which utilizes a more sophisticated electron-optic system to produce a higher signal-to-noise ratio than the orthicon, overcoming the beam-noise problem. The device, however, is relatively complex, expensive, and fragile.
The SIT (silicon intensifier target) and the SEC (secondary electron conduction) image converters employ a photocathode as the image sensor; the photoelectrons given off are focused onto a special target which provides high gain. In a SIT converter the target is a thin silicon wafer upon which a tightly spaced matrix of p-n junction diodes is formed. When the photoelectrons impinge onto the target, they cause multiple disassociations of the electron-hole pairs. The holes are collected at the p-side of the diode where the charge is neutralized by the scanning electron beam and the signal is read out on the backplate of the target. The SEC converter uses a thin layer of semiporous KCl which provides gain by internal secondary electron emission and the signal is also read out from the backplate of the target. These systems, like the others mentioned, require scanning electron beams and have their associated disadvantages.
The image dissector employs a photocathode light sensitive surface. The electrons given off are focused onto an image plane. A set of deflection coils provide fields which sweep the entire electron image across an aperture positioned near the center of the image plane. An electron multiplier acts on only those electrons passing through the aperture. The output signal is taken from the electron multiplier. This device, though of extremely high speed, has very low efficiency.
Charged coupled devices or photoresistive devices are solid-state systems requiring no scanning beam. Electron-hole pairs are created when light impinges on the p-type silicon imaging area. The charges, representing picture element signals, are stored in potential wells under depletion biased electrodes. The charges are transferred by applying a positive pulse to adjacent electrodes which are at right angles to the p-type channel stops. The whole image is thus transferred to a storage raster. Each horizontal line is then read out from the storage raster in sequence to provide the output signal. These devices, though fairly simple, are only moderately fast and not as sensitive as required for certain applications.
The purpose of the present invention is to provide a simple, low cost image converter having high sensitivity and speed, based on photocapacitive principles using low carrier concentration semiconductors and coherent detection, high capacitance insulators.
Therefore, it is an object of this invention to provide a solid state, photocapacitive image converter with high speed and sensitivity.
Another object of this invention is to provide an image converter that is simple and inexpensive to construct.
A further object of this invention is to provide an image converter which can be constructed so as to operate in a wide range of radiant energy wavelengths and magnitudes including infrared and low light as well as full sunlight and artificial light.
Yet another object of this invention is to provide an image converter which can be used in the presence of high magnetic fields.
Still another object of this invention is to provide an imaging device utilizing thermal capacitive principals.
A still further object of this invention is to provide a device that operates at room temperature and without an external biasing voltage.
Other objects and advantages of this invention will become apparent to those skilled in the art hereinafter in the specification and drawings.